(a) Field of the Invention
The present invention relates to Boost Converter for power factor and harmonic correction circuit of Switching Power Supply.
(b) Description of Prior Art
In FIG. 3 showing a conventional boost-chopper type converter in parallel operation, which is for example disclosed in Japanese Patent Laid-open No. 2-269469, reference numeral 1 designates A.C.power source, while reference numeral 2 designates a rectifier circuit in each converter unit 3,3A which can full-wave rectify A.C.input voltage from said A.C.power source 1 in order to output D.C.input voltage Vin, thus D.C.voltage source 4 can be constructed by said A.C.power source 1 and rectifier circuit 2.
To the both output terminals of said rectifier circuit 2 is connected a series circuit comprising inductance 6 and switching element 7 (or MOSFET) across resistor 5 for current detection, whereas to the both terminals of said switching element 7 is connected a series circuit comprising diode 8 and smoothing capacitor 9, of which the opposite terminals on the output side of said diode 8 are further connected to output terminals +V, -V respectively.
Then, on the basis of the difference between the current detection signals detected by resistor 5 and the voltage detection signals obtained by dividing D.C.output voltage Vout between said output terminals +V and -V, control circuit 10 can control the pulse conducting width for said switching element 7, thus preset D.C.output voltage Vout can be supplied to common load 11 across the output terminals +V, -V.
In aforesaid serial switching operation of the switching element 7, D.C.input voltage Vin is applied to inductance 6 during turning-on of the switching element 7, so that inductor current IL flowing through the inductance 6 ramps up, being followed by charging energy in said inductance 6. Whereas, during turning-off of said switching element 7, said energy charged in the inductance 6 is delivered to said smoothing capacitor 9 on the output side of diode 8, together with D.C.input voltage Vin from the rectifier circuit 2, with inductor current IL ramping down, and thus, D.C.output voltage Vout higher than D.C.input voltage Vin can be obtained.
At that time, the switching element 7 is on-off controlled by control circuit 10 so that said input current Iin from rectifier circuit 2, having average value of said inductance current IL, may take the full-wave rectified waveform proportional to D.C.input voltage Vin, whereby the equivalent to pure resistance load for said A.C.power source 1 can be obtained along with the improvement of power factor.
However, the above conventional art has a few problems described below;
In parallel operation, because of the voltage drop difference caused between output terminals +V,-V of converter units 3,3A and load 11, load currents I1 and I2, flowing into discrete converter units 3,3A across discrete output terminals -V, are caused to be inconsistent with each other. Especially in the case of boost converter shown in FIG. 3, each load current I1 and I2 flows through the rectifier circuit 2 to be fed back to common A.C.power source 1 on the input side, thus output balance of each converter unit 3,3A is inevitably lost or broken, therefore, stable control cannot be performed.